Download Ageing Alarm due to sudden variation of the O2- and CO2

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Oxidation Ageing Alarm due to sudden variation of the O2- and CO2-level in a
transformer
Dipl.Ing. Altmann, ARS-ALTMANN
1.
Introduction
The Oxidation Ageing Alarm is most often induced by the sudden variation of one-shot DGA
readings of the O2-level and the CO2-level in the transformer:
o
O2-level sinks from the previous quasi-steady-state level 16 – 20 000 ppm at 7 – 10 000
ppm
and consequently
o
CO2-level increases from 2 - 4000 ppm at 4 – 8000 ppm.
and
o
during the following months another one-shot DGA reading shows a full recovery of
O2- and CO2 –levels at approx. the same levels as before.
The most frequent explanation based on the standard DGA diagnostics interprets this fact as
a sudden and dangerous change of the intensity of the oxidation ageing of the cellulose
insulants of the transformer.
This conclusion should be regarded with the legitimate scepticism, because the main
boundary conditions necessary for the mentioned process, e.g. an abrupt and a strong
increase/decrease of the transformer load and the temperature is mostly not observed. The
transformer load and temperature usually varies, but the average temperature of the
transformer during a given period remains approx. the same.
Moreover, already the premise that the intensity of the cellulose oxidation in a transformer can
simply and without any reason abruptly increase and decrease is highly questionable.
2.
Flow-dynamic paradigm
The more plausible explanation of this effect offers the new paradigm based on the change of
the transportation of gases in the transformer.
This paradigm consequently considers the oil content of the main tank as a capacity with:
o
a continuous inflow of the O2 from the surrounding,
o
a continuous consumption of O2 due to the ageing process in the cellulose
o
and a continuous outflows of ageing products (CO and CO2) from the oil capacity back
into the surroundings.
All gases are under normal conditions diluted in the oil and its transport is therefore
performed only by the oil throughflow between the main tank and the conservator (and of
course by the difusion with the surroundings) .
The new paradigm then offers quite a different and a lot more convincing explanation of a
observed phenomenon.
The observed simultaneous drop / increase of the O2-level and the increase /drop of the CO
and CO2-level is induced by the decreasing / increasing troughflow of oil between the main
tank and the conservator, whereas the ageing intensity remains approximately the same.
For example, if the throughflow of the oil between the two tanks decreases due to small
temperature difference between the oil in the main tank and the oil in the conservator, then by the same intensity of the oxidation aging of the cellulose - the O2-level in the main
tank inevitably sinks and CO a CO2-level inevitably rises.
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Any variation of boundary conditions of the transformer which control the throughflow e.g. the
deviation of the load or the surrounding temperature therefore has to:
o
immediately
conservator
change the throughflow of the oil between the main tank and the
o
gradually change the DGA reading of O2 , CO and CO2-levels
The present DGA diagnostics, based on the comparison of historic change of absolute gas
levels under quasi-stable conditions, cannot recognize whether the change of O2- and CO,
CO2-levels corresponds to the change of the oxidation intensity or the change of the oil
throughflow between the main tank and the conservator.
3. The verification of the new paradigm
The basic statement mentioned above must be of course verified by a suitable and
plausible experiment under a normal operational conditions of a transformer.
a
For this goal the typical oven, a highly and constantly loaded, transformer was selected , with
the free oil level in the conservator provided with the continuous reading O2- and CO2-levels
in the oil filling in its main tank by the on-line chromatograph TGM Gatron (See
www.gatron.de)
The best demonstration of the
direct relationship between the O2
and CO2 – levels in the main tank
and
the
gas
transportation
between the main tank and the
conservator, is given by the
dynamic response of the present
system to the jump-like change in
the
transportation of gases
between the two tanks.
Valve 1
It can be done relatively easily in
this
case,
because
this
transformer is equipped with the
TRAFOSEAL II - See Fig. 1,
sealing function of which can be
easily switched on and off.
Valve 2
The TRAFOSEAL II represents
here an ideal tool for our
experiment – it enables a free
dilatation of the oil filling (the
transformer operation is not
negatively affected at all), but
effectively prevents mixing of the
protected oil in the main tank with
the oil from the conservator.
Fig. The schematic lay-out of the TRAFOSEAL II – the sealing function ON (Valve 1- closed,
Valve 2 – full open)
The desired jump-like change of the gas transportation between the main tank and the
conservator is then achieved by the simple opening / closing of the two slide valves:
the first regime represents the normal throughflow of the oil between the main tank and
the conservator - the TRAFOSEAL II is switched OFF, Valve 1 is open and Valve 2 is
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closed (See Fig.1) - the oil filling of the main tank directly communicates with the
conservator (and with the surrounding air as well).
the second regime simulates an effectively obstructed gas transport between the two
tanks, the TRAFOSEAL is switched ON, Valve 1 is closed and Valve 2 is open (See
Fig.1) - the input and output of the gases in the main tank is effectively obstructed due
to strongly reduced mixing of both oils in the main tank and the conservator.
For more detailed description of the function of TRAFOSEAL II See www.ars-altmann.com\
Product Range\ TRAFOSEAL \Technical Specification.
Results of the experiment are shown at Fig. 2.
TRAFOSEAL II
ON
O2, N2 (ppm)
70000
7000
60000
6000
50000
5000
40000
4000
30000
3000
CO2,CO,H2(ppm)
O2(ppm)
20000
2000
10000
1000
H2(ppm)
N2(ppm)
0
8.1.2005
0
27.2.2005
18.4.2005
CO2(ppm)
CO(ppm)
7.6.2005
t(days)
Fig.2 The on-line reading of the O2, N2, CO,CO2 and H2-levels in the main tank
The Fig.2 clearly shows the strong impact of the jump-like change in the boundary conditions
evoked by switching the TRAFOSEAL II on.
The ingress of air gases into the main tank and simultaneously, the escape of ageing products
from the main tank, is effectively obstructed and consequently:
•
the N2-level which previously increased, is constant now (the N2 as an inert gas serves
here as an ideal marker of the intensity of the transportation process between the main
tank and the conservator).
•
the O2-level permanently sinks - oxygen is intensively consumed in the ageing process,
until the non-measurable low oxygen content in oil (O2→0) is reached
•
CO2- and CO-levels permanently grow up till the oxygen (diluted in the oil filling of the
main tank) is completely consumed, then both levels remain constant.
The flow-dynamic interpretation of the experimental data is clear and simple – the ageing
process caused by the oxidation is de-facto stopped.
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This simple and plausible explanation of our experimental data must be fundamentally
inconsistent with the common interpretation based on two samplings of the oil from the main
tank:
•
the first sampling before the TRAFOSEAL action (e.g. at 10.5 2005) gives us relatively
high O2-content (20 000ppm) and relatively low CO2-content in the oil (2 800ppm)
•
the second one, e.g. after cca 5 monts ( at 8.5 2005), gives us an extremely low O2content (O2 < 100ppm) and high CO2-content in the oil (over 4 500ppm).
The common DGA interpretation of the same process is then a massive increase of a the
oxidation ageing process in the cellulose.
4. Conclusion
Our experiment clearly shows an essential drawback of the present DGA diagnostic method.
This method works only with absolute levels of gases in the oil and a-priori excludes the effect
of the flow dynamics in the examined system at these levels.
Two absolutely different interpretations of our experiment show how misleading the present
DGA diagnostics can be:
•
common DGA diagnostics obviously wrongly predicts a massive increase of the
oxidation ageing of the cellulose
whereas
•
new flow dynamic paradigm legitimately claims that the ageing caused by oxidation
of the cellulose materials is de-facto stopped
The jump-like change of boundary conditions in our experiment is of course purposely
extreme to show a not only quantitative but also qualitative discrepancy in interpretations.
The comparable obstruction of the gas transportation in the ordinary transformer (no or very
weak throughflow of the oil between the main tank and the conservator) can be achieved (e.g.
by the constant temperature of the oil in the main tank, the low temperature difference
between the main tank and the conservator, the long tube connecting both tanks, …), but
only for a relatively short time and therefore the real change in the O2-, CO- and CO2readings is not so dramatic.
The present DGA diagnostics of the oxidation ageing reliably indicates, whether the notdesired process in the transformer exists or not, but cannot determine what amount of the
specific gas [ in m3 s-1 or kg s-1 units ] produces an examined fault.
The present DGA diagnostics is and remains a very good first (and easily accessible)
indicator of the oxidation ageing of a transformer which should give us an impulse to start a
proper diagnostic method.
Generally, the plausible evaluation of faults in the transformer must be based on:
•
jump-like change of boundary conditions of a transformer
•
on-line reading of fault gas levels in the main tank (and in the conservator)
•
quantitative evaluation of a fault by the amount of consumed and/or produced specific
gas
For more information about a corresponding DGA gradient method See
altmann.com \News\ Impact of vacuum treatment on DGA.
www.ars-